<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <style>
      .mask{
    background-color: black;
    position: fixed;
          left: 0;
          right: 0;
          top: 0;
          bottom: 0;
          opacity: 0.7;
            border:1px solid 255,255,255}

.c1{
}
      }


  </style>
</head>
<body>

<div class="c1"></div>



</body>
</html>